1. Field of the Invention
The present invention relates to a process for phosphating steel strip, which has been galvanized electrolytically and/or by melt immersion, in which process zinc phosphate layers containing manganese and nickel are formed. These zinc phosphate layers containing manganese and nickel are applied by spraying, spray-immersion and/or immersion with aqueous solutions.
2. Statement of Related Art
Processes for phosphating surfaces of iron, steel, zinc and the alloys thereof as well as of aluminum have long been state of the art (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Volume 15, Pages 686 and 687). Phosphating said surfaces serves to increase the adhesion strength of paint layers and to improve the protection from corrosion.
From W. A. Roland and K. -H. Gottwald, Metalloberflache, 42nd Year 1988/6 there have been known manganese-modified zinc phosphate coatings as adhesion primers for modern paint coatings. In this reference it is set forth that the use of manganese ions besides zinc and nickel ions in low-zinc phosphating processes, especially when using surface-modified thin sheets, demonstrably improves the anticorrosive property. The incorporation of manganese in the zinc phosphate coatings results in smaller and more compact crystals having an increased alkali resistance. At the same time the working range of phosphating baths is extended; aluminum also can be phosphated in a composite with steel and steel which has been galvanized electrolytically or by melt immersion to form a layer, with the generally achieved quality standard being ensured.
From DE 32 45 411 A1 there has been known a process for phosphating electrolytically galvanized metal goods, and especially galvanized steel strips, by short-term treatment with phosphating solutions which may contain, in addition to zinc and phosphate ions, further metal cations and/or anions of oxygen-containing acids having an accelerator effect. In these processes zinc phosphate layers having an areal density of less than 2 g/m.sup.2 are formed. Acidic phosphating solutions, the content of Zn2+ cations of which is from about 1 to 2.5 g/l, are employed, while the free acid content is kept within the range of from 0.8 to 3 points, and the acid ratio of total acid to free acid is kept within the range of from 5 to 10. The duration of this treatment is not to be substantially more than 5 seconds.
It is preferred to work with nitrate-containing phosphating baths wherein the ratio by weight of Zn.sup.+2 /NO.sub.3.sup.- is maintained within the range of from 1:1 to 1:8 and the ratio by weight of PO.sub.4.sup.-3 /NO.sub.3.sup.- is maintained within the range of from 1:0.1 to 1:2.5.
From EP 0 106 459 A1 there has been known a phosphating process wherein zinc phosphate coatings containing nickel and manganese are formed. The presence of fluoride ions is considered as being essential, and so is the upper concentration limit of 10 g/l of nitrate anions.
From EP 0 112 826 B1 there has been known a high-nickel phosphating process. Herein, a molar ratio of nickel to zinc within the range of from 5.2:1 to 16:1 is required.
Moreover, from EP 0 175 606 there has been known a phosphating process wherein, more particularly, the use of iron-containing phosphating baths is featured. Furthermore, a number of organic substances are used as accelerators, whereas the presence of manganese is not necessary. In addition, definite ratios of zinc to nickel and of zinc to iron are required.
The processes as presently used in practice for phosphating steel strip which has been galvanized electrolytically and/or by melt immersion are still subject to limitations, the elimination of which is desirable. Thus, In order to ensure a sufficient protection from corrosion, it is deemed to be required to form areal densities of the phosphate coatings of less than 2 g/m.sup.2. An unsatisfactory to poor adhesion of subsequent coatings frequently is the result of a comparatively high areal density, more particularly if phosphated and coated material is deformed. In the processes utilized in practice, the duration of phosphating conventionally is more than 2 seconds, especially with strip velocities of about 60 to 120 m/min.
It has been known that, by a use of nickel cations in the phosphating solutions, improved values of adhesion and of protection from corrosion can be achieved. However, under this aspect it has been observed in the course of elaborating the present invention that an increase in the nickel concentration which results in an improvement of the anti-corrosive property at the same time causes a dark coloration of the zinc phosphate layer containing manganese and nickel.